This invention relates to sewing machines and, more particularly, to sewing machines utilizing stitch pattern signal responsive actuators for positioning the needle and/or the work feeding mechanism between successive stitches of a predetermined pattern.
Sewing machines which automatically stitch a preselected pattern are well known and have been in use for a number of years. In the past, these machines have typically been operated by providing a selected pattern cam for controlling the positioning of the needle and/or the work feeding mechanism in accordance with a desired pattern of stitches. Such cams are operatively connected to the main drive motor of the machine and hence there is an inherent synchronization between the positioning mechanism and the penetrations by the needle to form the stitches. However, with the advent of electronically controlled sewing machines, the pattern cams have been replaced by actuators responsive to stitch pattern signals stored in a memory. Insofar as speed of operation is concerned, these actuators operate imdependently from the main drive motor of the machine and hence a problem can arise if the operation of the actuator is not completed prior to repenetration of the needle. Typically, these actuators operate at a fixed speed. Therefore, the time it takes to position the needle and/or the work feeding mechanism is dependent upon the distance that must be traversed. Since the positioning must be complete between successive needle penetrations, as the stitch rate of the machine increases, less time is available for the positioning. In a machine utilizing stepping motors as the actuators, the required positioning time may be greater than is available, based on the rotational speed called for by the machine operator. When this situation occurs, patterning errors can result, for example, zig-zag width may decrease as the sewing speed increases, and the stepping motor may even be pulled out of pole orientation.
It is therefore an object of this invention to insure sufficient time for the actuator to reposition the bight or feed mechanism.